You probably know that owls can rotate their heads a remarkable 270 degrees in either direction. It's practically their trademark — but it's a move that by all measure should result in a massive stroke or embolism, the result of torn blood vessel linings and the onset of fatal clots. Nobody's understood the physiology that lets owls do this — until now.

Finally, a collaboration between medical illustrators and neurological imaging experts has yielded the answer — and it has to do with the owls' remarkable bone structure and vascular network.

Necks aren't supposed to be able to move the way owls' do, as many studies on humans and other animals have proved. Tests on people who have suffered from sloppy chiropractic adjustments, whiplash, and even jarring roller coaster rides show that such sudden head and neck gyrations can be extremely damaging (if not fatal). These extreme contortions can cut off blood supply and damage fragile blood vessels.

Looking to figure out how owls are able to pull off such a remarkable feat, a multidisciplinary team from Johns Hopkins collaborated on a study that's set to appear in the February 1 issue of Science. Led by medical illustrator Fabian de Kok-Mercado and neuroradiologist Philippe Gailloud, the team has proposed that several distinct biological adaptations make the extreme twist possible — and they all have to do with the bone structure and vascular network that supports the owl's top-heavy head.

To conduct the study, the researchers used angiography and CT scans when examining the heads and necks of snowy, barred, and great horned owls, all of whom died from natural causes. In order to physically see what was actually happening inside the owls' heads, they injected a dye that enhanced X-ray scans of the blood vessels being studied.

Then, by mimicking blood flow, the scientists could see that blood vessels at the base of the head (just under the jaw bone) were getting increasingly larger as the dye was introduced, but before the fluid pooled in reservoirs. There is no equivalent to this in human anatomy; in fact, human arteries tend to shrink as they branch out — and they certainly don't balloon out.

The researchers surmise that these expandable blood reservoirs are what enables the owls to pool their blood, an adaptation that allows them to make radical gyrations of the head — and all without cutting off their energy supply. In turn, the supporting complex vascular network minimizes interruption in blood flow.

Among the anatomical variations discovered were bony holes in the vertebrae — hollow cavities in the neck where a major artery feeds the brain. These were about 10 times larger in diameter than the vertebral artery traveling through them. This extra space in the transverse foraminae (the holes surrounding the vertebral arteries) provides owls with a set of cushioning air pockets that allow the artery to move around when it's twisted. And in fact, 12 of the owl's 14 cervical vertebrae have this feature.

And interestingly, the scientists also discovered that the owl's vertebral artery enters the neck higher up compared to other birds. Normally, it's the 12th cervical vertebrae, but for the owl, it's the 14th (thus allowing for more vessel room and slack).

Another non-human-like feature included small vessel connections between the carotid and vertebral arteries that allow blood to be exchanged between the two blood vessels. This allows for uninterrupted blood flow to the brain — even in the event that a route is blocked during extreme neck rotation.

In future, the researchers will study hawks to see if they have similar features.